US 6845212 B2
An optical element, such as a lightguide or an optical film, is formed with a predetermined, programmed pattern of optical structures. The optical structures may be arranged to selectively correct for non-uniformities in the output a lightguide, or may be arranged to otherwise enhance the performance of a display.
1. An optical film for use with an edge illuminated lightguide having an input edge surface and an output surface, the optical film comprising:
a first surface;
a second surface opposing the first surface;
a plurality of prism grooves formed in the first surface, the prism grooves each having a groove axis and each groove axis being substantially parallel to each other groove axis, wherein the groove axes are arranged to be disposed substantially perpendicular to the input edge surface; and
each of the plurality of prism grooves being formed to include a plurality of optical structures, the plurality of optical structures to provide optical power to the prism groove, each optical structure having a characteristic, and the characteristic varying as a function of the location along the groove axis from the input edge surface.
2. The optical film of
3. The optical film of
4. The optical film of
5. The optical film of
6. The optical film of
7. The optical film of
8. The optical film of
9. The optical film of
10. An optical film comprising a surface, a first edge and an opposing second edge, grooves formed in the surface extending from the first edge to the second edge, each groove having a groove axis and the groove axes being substantially aligned, optical structures formed on each of the grooves, the optical structures providing optical power to the grooves, and the optical structures having a characteristic that varies along the groove as a function of the location of the optical structure on the groove between the first edge and the second edge.
11. The optical film of
12. The optical film of
13. The optical film of
14. The optical film of
15. The optical film of
16. The optical film of
17. The optical film of
18. The optical film of
19. A method of reducing defects in a light output of a backlight, the method comprising:
providing an optical element in the back light system, the optical element having a surface, a first edge and an opposing second edge, grooves formed in the surface extending from the first edge to the second edge, each groove having a groove axis and the groove axes being substantially aligned, optical structures formed on each of the grooves, the optical structures providing optical power to the grooves, and the optical structures having a characteristic that varies along the groove as a function of the location of the optical structure on the groove between the input edge and the second edge; and
arranging the optical element such that the grooves are aligned substantially perpendicular to a light source of the backlight and in an output path of the light source.
1. Field of the Invention
The invention relates generally to optical elements and more particularly to lightguides, optical films and other optical elements suitable for use in display devices and having programmed optical structures.
2. Description of the Related Technology
Backlit display devices, such as liquid crystal display (LCD) devices, commonly use a wedge-shaped lightguide. The wedge-shaped lightguide couples light from a substantially linear source, such as a cold cathode fluorescent lamp (CCFL), to a substantially planar output. The planar output is then used to illuminate the LCD.
One measure of the performance of the backlit display is its uniformity. A user can easily perceive relatively small differences in brightness of a display from one area of the display to the next. Even relatively small non-uniformities can be very annoying to a user of the display.
Surface diffusers or bulk diffusers, which scatter the light exiting the lightguide, are sometimes used to mask or soften non-uniformities. However, this diffusion also results in light being directed away from a preferred viewing axis. A net result can be a reduction in overall brightness of the display along the preferred viewing axis, which is another performance measure of a display device.
Unlike non-uniformities, from a subjective standpoint relatively small increases or decreases in overall brightness are not easily perceived by the user of the display device. However, the display device designer is discouraged by even the smallest decreases in overall brightness including decreases so small they might only be perceived by objective measurement. This is because display brightness and power requirements of the display are closely related. If overall brightness can be increased without increasing the required power, the designer can actually allocate less power to the display device, yet still achieve an acceptable level of brightness. For battery powered portable devices, this translates to longer running times.
In accordance with the invention, an optical element, such as a lightguide or an optical film, is formed with a predetermined, programmed pattern of optical structures. The optical structures may be arranged to selectively correct for non-uniformities in the output of a lightguide, or may be arranged to otherwise effect the performance of the display in a predetermined, and designed manner.
In a first aspect of the invention, an optically transmissive film having a first surface and a second surface and a first edge and a second edge is formed with a plurality of optical structures formed in the first side. The plurality of optical structures are arranged on the first side in a predetermined pattern, and each optical structure has at least one characteristic selected from the group consisting of an amplitude, a period and an aspect ratio. Each characteristic has a first value for a first predetermined location on the film between the first edge and the second edge and the characteristic has a second value, different from the first value, for a second predetermined location on the film, different than the first predetermined location on the film, between the first edge and the second edge.
In another aspect of the invention, the structure in accordance with the invention is part of a thick optical element, such as for example, a lightguide wedge. The structure is achieved on the thick element through injection molding, compression molding, or by bonding a film with the structure to the additional optical element.
The many advantages and features of the present invention will become apparent to one of ordinary skill in the art from the following detailed description of several preferred embodiments of the invention with reference to the attached drawings wherein like reference numerals refer to like elements throughout and in which:
The present invention is described in terms of several preferred embodiments, and particularly, in terms of an optical film or a lightguide suitable for use in a backlighting system typically used in flat panel display devices, such as a laptop computer display or a desktop flat panel display. The invention, however, is not so limited in application and one of ordinary skill in the art will appreciate that it has application to virtually any optical system, for example, to projection screen devices and flat panel televisions. Therefore, the embodiments described herein should not be taken as limiting of the broad scope of the invention.
The light source 12 may be a CCFL that inputs light to the edge surface 21 of the lightguide 16, and the lamp reflector 14 may be a reflective film that wraps around the light source 12 forming a lamp cavity. The reflector 24 backs the lightguide 16 and may be an efficient back reflector, e.g., a lambertian film or a specular film or a combination.
In the embodiment shown, the edge-coupled light propagates from the input surface 21 toward the end surface 22, confined by total internal reflection (TIR). The light is extracted from the lightguide 16 by frustration of the TIR. A ray confined within the lightguide 16 increases its angle of incidence relative to the plane of the top and bottom walls, due to the wedge angle, with each TIR bounce. Thus, the light eventually refracts out of the output surface 18 and at a glancing angle thereto, because it is no longer contained by TIR. Some of the light rays are extracted out of the back surface 20. These light rays are reflected back into and through the lightguide 16 by the back reflector 24. First light redirecting element 26 is arranged as a turning film to redirect these light rays exiting the output surface 18 along a direction substantially parallel to a preferred viewing direction.
As shown in
Referring back to
With lightguides used for backlighting, such as lightguide 16, it is common for there to be non-uniformities in the light output from the lightguide. These non-uniformities can frequently be concentrated near the input surface 21. To mask these defects in applications of the lightguide, a diffuser that covers the output surface of the lightguide is typically used. However, a diffuser tends to reduce the overall brightness of the display and may not adequately mask all of the defects.
Referring now to
With continued reference to
It should be appreciated that using ray tracing and other analysis techniques, it is possible to determine particular arrangements for the optical structures 40 and the pattern 42 that best correct particular observed non-uniformities in the output of the lightguide 16. That is, one or more of the characteristics of the optical structures 40 and the pattern 42 may be tailored to correct a particular non-uniformity. As described above, in connection with first light redirecting element 26, the optical structures 40 and the pattern 42 provided optical power to the output of the lightguide 16 near the input surface 21 in order to mask non-uniformities that may occur near the input surface 21. Less or no optical power is provided away from the input surface 21 as fewer or less intense non-uniformities are typically observed from the lightguide 16 farther from the input surface 21. In this manner, optical power is provided where most needed to mask or soften non-uniformities, while less optical power is provided where there may be fewer non-uniformities to mask. Moreover, optical power may be added virtually anywhere to the output of the lightguide by adding optical structures and/or varying the characteristics of the optical structures. Furthermore, the addition of optical power need not be uniform. Instead, optical power may be added, as necessary, to discrete regions of the lightguide output if necessary to help mask a defect or create a particular optical effect.
Planar light guides, and some wedge light guides that operate using frustrated TIR, may include an extractor pattern on a back surface of the lightguide. Typically, the extractor pattern is a pattern of white dots disposed on the back surface of the lightguide. Light incident to one of the dots is diffusely reflected by the white dot, and a portion of this reflected light is caused to exit the light guide. In spite of the diffuse nature of this method of extracting light from the lightguide, the pattern of dots may itself be visible in the lightguide output. Thus, to hide the dot pattern, additional diffusion is typically provided.
With reference to
While so far discussed in terms of optical films, the invention has application to the lightguide wedge itself. Referring to
With continued reference to
With reference to
As will be more generally appreciated from the foregoing discussion, virtually any configuration of optical structures may be formed into an optical film, and the optical film coupled, for example by bonding, to a lightguide or other bulk optical element. For example, glare reduction, anti-wetout, Fresnels, and virtually any other structure that may be formed in a surface of an optical film may be easily replicated into the film and then the film coupled to another optical element.
Films incorporating programmed optical structures may be manufactured using a microreplication process. In such a manufacturing process, a master is made, for example by cutting the pattern into a metal roll, and the master is used to produce films by extrusion, cast-and-cure, embossing and other suitable processes. Alternatively, the films may be compression or injection molded or roll formed. A preferred apparatus and method for microreplication is described in the commonly assigned U.S. patent application entitled “Optical Film With Defect-Reducing Surface and Method of Making Same,” Ser. No. 09/246,970, the disclosure of which is hereby expressly incorporated herein by reference.
As an example of the above-described feature, and with reference to
Referring now to
With continued reference to
With reference to
In the lightguide 121 illustrated in
Still other modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. This description is to be construed as illustrative only, and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and method may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.